A tungsten jig Mop fly in chartreuse from Solitude Fly. The tungsten bead sinks it fast while the chartreuse mop body gives trout a bright, unmistakable target.
Fish it subsurface near the bottom under an indicator, especially when fish need a bold trigger.
Adjust your indicator depth until the fly is riding just off the bottom, where nymph-feeding trout do most of their hunting.